1. Field of the Invention
The present invention relates to an automatic transfer apparatus used in a machining system, such as a transfer unit of a transfer machine.
2. Discussion of Related Art
In transfer machines, automatic transfer apparatus of a so-called lift-and-carry type have been widely used. An example of such automatic transfer apparatus is shown in FIG. 1.
In the automatic transfer apparatus, a transfer bar 51 having positioning blocks 50 for receiving workpieces W is arranged for movement in a transfer direction and for up and down movement. One longitudinal end of the transfer bar 51 is engaged with a vertically extending guide plate 52 via a pair of guide rollers 53. Therefore, the transfer bar 51 is movable only in a vertical direction with respect to the guide plate 52. The guide plate 52 is slidably guided by a pair of guide bars 54 which are supported by a machine frame to be parallel with each other and extend in the transfer direction. Also, the guide plate 52 is engaged with a feed screw 56 which is arranged in parallel with the guide bars 54 and is driven by a servomotor 55 for transfer movement.
A transmission shaft 57 is disposed under the transfer bar 51 in parallel therewith for reciprocating movement in the transfer direction. To one end of the transmission shaft 57, a piston rod of a hydraulic cylinder 58 is connected. A suitable number of bell cranks 59 are disposed between the transfer bar 51 and the transmission shaft 57 at predetermined intervals for rotational movement. The lower ends of the bell cranks 59 are coupled with the transmission shaft 57, while support rollers 60 for supporting the transfer bar 51 are attached to the upper ends of the bell cranks 59.
In the above-described lift-and-carry type transfer apparatus, when a single machining operation at each machining unit of the transfer machine is completed, the transmission shaft 57 is moved by the hydraulic cylinder 58 to the left, as viewed in FIG. 1. Due to the movement of the transmission shaft 57, the bell cranks 59 rotate in the clockwise direction so that the transfer bar 51 is lifted by the support rollers 60, whereby workpieces W are lifted altogether. During this lift movement, the pair of guide rollers 53 move upward while rolling on the guide plate 52.
The feed screw 56 is then rotated by the servomotor 55 so that the guide plate 52 is displaced in the transfer direction by a distance corresponding to the pitch of the machining stations. This movement of the guide plate 52 is transmitted to the transfer bar 51 via the guide rollers 53 so that the transfer bar 51 is moved in the transfer direction by the distance corresponding to the distance between the midpoints of adjacent machining stations. As a result, the workpieces W are transferred to downstream machining stations. Subsequently, the transmission shaft 57 is moved by the hydraulic cylinder 58 to the right, as viewed in FIG. 1. With this operation, the bell cranks 59 are rotated in the counterclockwise direction so that the transfer bar 51 is lowered. As a result, the workpieces W are all seated on the workpiece supporting locations of the machining stations. Then, machining operations are carried out. After the transfer bar 51 is lowered to the lowest position, the feed screw 56 is reversely rotated by the servomotor 55 so that the transfer bar 51 is moved in a direction opposite to the transfer direction by the distance corresponding to the distance noted above between adjacent machining stations. As a result, the transfer bar 51 is returned to the original position.
In the above-described conventional transfer apparatus, the transfer bar 51 is designed to have a length substantially equal to the entire length of the machining line. Accordingly, the length of the transfer bar 51 must be a predetermined length corresponding to the number of machining stations.
Therefore, when the number of the machining stations in the transfer machine is changed, the transfer bar 51 of the transfer apparatus must be replaced with a different one having a different length to adapt the change in the entire length of the transfer line.
However, a considerably long time is required to replace the transfer bar 51 with a different one having a different length for changing the length of the transfer line. Accordingly, the transfer machine must be stop for a long time.